This invention relates to a method of improving the viscosity stability of aqueous compositions. The method of this invention is particularly applicable to improving the viscosity stability of aqueous compositions containing associative thickeners.
Associative thickeners are water soluble or water swellable polymers that have chemically attached hydrophobic groups. The associative thickeners operate to thicken systems to which they are added by the non-specific associations, such as adsorption on surfaces and aggregation in solution akin to micellization, between the hydrophobic groups on the thickener molecules and moieties on the other components in the system, similar to the non-specific associations of conventional surfactants.
Since the hydrophobic association exhibited by associative thickeners is non-specific, it is greatly influenced by the presence of surfactants and water miscible organic solvents. The hydrophobes of surfactants may compete for adsorption sites on particle surfaces, and can hinder or enhance associations between thickener hydrophobes, depending on the surfactant HLB. Once an associative thickener is completely desorbed from a particle, such as a latex particle, it can behave like a non-absorbing thickener and flocculate the latex by the volume restriction process.
A classic problem in paints containing associative thickeners is a drop in mid-shear (Krebs-Stormer) viscosity when colorants that contain high levels of surfactant are added. This is especially problematic when the paint will be tinted to a deep tone because a high level of surfactant generally accompanies the colorant. Colorants are added to paints in units of ounces of colorant per gallon of paint (hereinafter referred to as xe2x80x9coz/galxe2x80x9d). Light-tint (pastel) paints typically contain no more than 4 oz/gal of colorant. Mid-tone paints typically contain from greater than 4 oz/gal to 8 oz/gal of colorant. Deep tone paints typically contain 8 oz/gal up to 12 oz/gal of colorant.
Generally, it is possible to formulate a light tint base at a high enough mid-shear viscosity that colorants added to it will not depress the viscosity to an unacceptable degree. Combinations of associative thickeners have been found to be less sensitive to colorant addition than the individual thickeners alone, in some cases. However, neither of these solutions is completely satisfactory because they either require tedious reformulation, added cost or both, particularly in deep tone paints. The method and mixture containing a multiphobe and a monophobe of the present invention provide a solution to these problems.
Combinations of multiphobe, such as an associative thickener, with a monophobe, such as a surfactant, are known in many conventional aqueous compositions, such as, for examples in paints, adhesives and other coating compositions. For example, EP-A-0 773 263 discloses a substantially anhydrous composition containing a solid associative thickener and one or more surfactants and specifically notes on page 3, lines 47-48 that the mixture disclosed therein is inferior in pigmented coating compositions. Tarng et al., xe2x80x9cModel Associative Aqueous Solutions,xe2x80x9d Adv. Chem. Ser., Volume 48, pages 305-341 disclose mixtures of HEUR associative thickener with low molecular weight surfactants, namely either anionic (sodium dodecylbenzenesulfonate, MW=348) or nonionic (C13H27(OCH2CH2)9OH, MW=596) surfactants.
None of these conventional aqueous compositions utilize a mixture of a select multiphobe component and a select monophobe component, wherein the ratio of the molecular weight of the monophobe to molecular weight of the multiphobe is relatively large in comparison to similar components in conventional aqueous systems. The unique mixture of the invention provides a solution to a serious problem that has eluded the paint industry for more than 20 years.